Fiber nonlinearity is usually the limiting factor in determining the transmission distance of optical communications. One of the ways to mitigate fiber nonlinearity is to use multi-subcarrier (MSC) modulation. It has been theoretically and experimentally demonstrated that MSC modulation comprising of 2-4 GB subcarriers is the best in terms of nonlinearity tolerance. This is due to the fact that coherence among subcarriers decreases with narrower subcarriers.
Four-dimensional (4D) constant modulus modulation formats have also been shown to have nonlinearity tolerance compared to conventional modulation formats in the conventional 31-34 GBd wavelength domain multiplexed (WDM) channels.
In this invention, we apply 4D constant modulus formats to MCS. Mapping 4D constant modulus signal comprising two complementary amplitude signals onto dual polarization optical signal has multiple options. The pair can be mapped across two polarizations of the same time slot, across the two time slots of the same polarization, or across two subcarriers of the same time slot and the same polarization. In particular, in the case of MSC, the third option becomes a viable option, since they can be generated from a single transmitter. On the other hand, conventional single carrier transmitter cannot offer this option.